The investigators'work to date has identified two dynamic mesodermal domains in the developing vertebrate embryo. The primaxial domain is populated exclusively by cells from the somites. The abaxial domain is made up of myoblasts from the somites and connective tissue of the lateral plate in which the muscles differentiate. They have termed the boundary between the two domains the lateral somitic frontier (LSF). This new terminology avoids the confusion caused by the functional terms epaxial/hypaxial when referring to embryonic populations. The investigators have partially mapped the LSF in the chick using quail-chick chimeras and more recently mapped the frontier in the mouse (Durland et al., 2008). These glimpses of the frontier revealed abaxial/primaxial boundaries that are cryptic in adult morphology. Interestingly, these cryptic domains are consistent with the distribution of the impact of different natural and experimental mouse mutants. It is hypothesized that the LSF is the site of early information exchange that is crucial for normal patterning of the vertebrate body. The work proposed here will extend our knowledge about the LSF into the common amphibian model, the frog (Xenopus) and salamander (Ambystoma-axolotl) systems. Taking advantage of new GFP transgenics in both frog and salamander species, the investigators will first use isotopic transplants to map the somitic-lateral plate interface in these anamniotes, and second, use heterotopic somite transplants to test the hypothesis that the frontier represents a patterning boundary. Amphibians offer distinct advantages for direct experimental approaches. The long-term goal of this project is to establish the GFP transgenic amphibians as systems for testing hypothesis about tissue interactions during important embryonic events. Medical science depends on the validity of extrapolating experimental results among model systems. The advantages of amphibian species for studying body wall formation include the relative simplicity of the thoracic and abdominal body wall relative to amniote systems, the large cell size facilitating identity of mixed lineage populations, and the abundant molecular and developmental techniques already available in Xenopus. A fuller understanding of the dynamic interactions among mesodermal populations in all of our common model systems will increase the relevance and value of experimental results. NARRATIVE: This work investigates body wall formation in vertebrate embryos. Errors in the complex interactions of the embryonic tissues that form the ribcage and body wall result in birth defects in the human population. The proposed experiments seek to understand the dynamics of mesodermal tissue interactions using morphologically simple amphibian model systems.